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240 Volt Notes

These are notes about deciphering the particulars about the 240V Service in my workshops so that I could wire a new piece of equipment.


I'm not an electrician and have done only a small amount of wiring work, and an even smaller amount of 3 phase wiring work. If you believe anything you read in this note and act upon it, you'll likely electrocute yourself or burn down the home that you spent thirty years saving for. You've been warned...


Electrical diagrams showing power from the power company often label the leads as L1 (Line 1), L2, and L3, or as La (Line A), Lb, and Lc. These are the same thing. The number of leads you have depends upon the type of service you obtain from the Power Company (POCO). In addition to L1, L2, and L3, your service may or may not include a Ground (G) wire or a Neutral (N) wire.

The service entrance will feed into a Load Center or Breaker Panel (BP) (they are the same thing). This is where your circuit breakers are located. Inside of the BP are busses referred to as A, B, and C. You'll only have a C, i.e., a third bus, if you have three phase service. L1, L2, and L3 from the POCO will connect to A, B, and C, respectively. Circuit Breakers (CB) snap in over the busses and pick up their electricity from them, which the breakers conduct to the wires attached to them.

Wires from the CB carry electricity to a load or to a receptacle. Contacts on receptacles and plugs are labeled as well. X, Y, and Z are used to indicate contacts that are "hot". Typically, electricity flows from A, B, and C, through the CB and wire to the X, Y, and Z contacts in the plug/receptacle. Similarly, G and N from the BP may also flow through wire (no CB is inline for these) to G and N (respectively) on the plug/receptacle. However, it is important to note that not all plugs/receptacles have G and/or N contacts. There is a huge number of plugs/receptacles on the market with a huge array of configurations.

Three phase electricity comes in many flavors. It seems complicated enough that many people on the internet (who claim to be electricians) get it wrong -- frequently. Be careful here - tread lightly - believe nothing until you have seen it confirmed in half a dozen legitimate sources. I'm not kidding here. I suspect that part of the problem is that most ordinary residential electricians simply don't see a lot of 3 phase AC. Even within a geographic region, more than one type of 3 phase may be in use. When viewing the US as a whole, there's even more variety. It's a complicated topic.


The service to the new workshop is three insulated wires and one bare wire. This is known as 4 Wire Three Phase service. Inside the BP, there are three hot busses (A, B, and C) and one neutral bus (G/N). The incoming service wires connecting to A and B are black. The incoming service wire connecting to C is black as well, but it is also wrapped with red tape. The red tape denotes the high leg (more about this shortly).

Voltage readings across A-B, B-C, and A-C are all 250v. Voltage across A-G/N, B-G/N, and C-G/N are 124v, 124v, and 214v, respectively. From this, we know that the incoming service is 4 Wire Three Phase High Leg Delta (with Grounded Neutral). Diagrammatically:

/-------------------------- L1    Service from POCO
|      /------------------- L2
|      |      /------------ L3
|      |      |      /----- G/N
A      B      C      |
|      |    (RED)    |
|      |      |      |
| 250v |      |      |
|<---->| 250v |      |
|      |<---->|      |
| 250v |      |      |
|<-----+----->|      |
|      |      | 214v |
|      | 124v |<---->|
| 124v |<-----+----->|
| <----+------+----->|
|      |      |      |

The panel was installed in the 70's by an industrial electrician. At that time, the NEC stated that the high leg was to be connected to the C bus. By the end of the decade, the NEC was changed to require the high leg to be connected to B. Systems wired to the earlier specification are acceptable and can continue to be used, modified, and added on to. It is important to be aware of this change, as any documentation or instructions created in the past 30 years will expect B to be the high leg, so one must adjust things accordingly.

In high leg delta service, one leg is center tapped at the transformer, which accounts for its different potential relative to neutral. This service is also called red leg or stinger, depending upon the part of the country one is in.

Adding a L6-30R

With all that out of the way, here are notes about adding an L6-30R receptacle in my new shop.

After moving a belt/disk sander to the new shop, I discovered that I needed to install a L6-30R (250v, single phase, grounded) receptacle. An existing 30 amp 3 phase breaker in the BP fed to a three pole Safety Switch (SS), to which a 10hp 3 phase cutoff saw was already connected. Because I would never use both the cutoff saw and the belt/disk sander at the same time, I wanted to mount the L6-30R to the SS for the cutoff saw, wiring the L6-30R to the output side of the SS. This allowed the existing breaker to be used by either piece of equipment (but not both at the same time).


I had to do a little reading to determine how to wire the receptacle. The L6-30R has terminals for X, Y, and G. In other words, it isn't a three-phase connector, but that's fine, as the equipment being added isn't three-phase either.

Because A, B, and C are not all at the same potential relative to ground, one must exercise care in choosing the hot conductors to wire to the receptacle. In this situation (i.e., a system wired according the 70's NEC), A should be wired to X, B wired to Y, and G/N to G. C, the high leg, is left unused (after all, this isn't a 3 phase motor or a 3 phase plug/receptacle).

The above conclusion was confirmed by examining existing three-phase wiring in my old workshop. Because the shops are adjacent, I assumed that the type of service was identical.

I studied a 6-50 receptacle feeding out of a SS (also installed by an electrician). The 6-50 provides two hot connections and one ground connection (i.e., X, Y, and G). The L6-30R provides the same connections, although at 30 amps and in a different form factor. Examining the wiring of the 6-50 revealed that A was wired to X, B to Y, and G/N to G. This led me to believe that the above idea for the wiring of the L6-30R was correct. Spoiler - it wasn't - so keep reading.

Sanity Check

Armed with this knowledge, I wired the L6-30R similarly to the 6-50. It was time to see if the results made sense. The readings from a DMM were: X-G 124v, Y-G 214v, X-Y 250v. X-Y, which should be A-B, made sense at 250v. X-G, which should be A-G/N, made sense at 124v. However, Y-G, which should be B-G/N, was 214v which looked wrong -- shouldn't it have been 124v? What's up?

In the new shop, the main service feeds into the load center, as previously described. There is a 3 phase 30 amp breaker, which feeds out to an attached SS. I wired the L6-30R receptacle to the output side of this SS. In tracing things back to the breaker, I discovered that the B and C conductors from the breaker were swapped at the SS. I had simply assumed that A, B, and C in the load center went to A, B, and C, respectively, in the SS. I don't know what the code says, but my assumption seemed logical at the time. However, if a three phase load is attached to the output of the SS, it really doesn't matter which leg goes where, as the voltage between all three legs is 250v. The exception to this might be (I think) if the direction of motor rotation was incorrect, in which case, two legs would have to be swapped (again, so I think).

At least w.r.t. the cutoff saw, I would guess that the order didn't matter. Surely, if it had been wired one way, but two legs had to be swapped to reverse rotation, the electrician would have done so on the output side of the SS, not on the input side, where the breaker in the load center feeds into the SS input. This is all uneducated speculation anyway.

The bottom line is that I did find the explanation for the unexpected voltages. How to resolve the problem? Two options presented themselves. One, would be to swap B and C inputs to the SS. The other would simply be to wire X and Y in the L6-30R to A and C, rather than A and B. The former seemed misguided, as it is based only upon a guess on my part, and it could also require wiring changes at the cutoff saw's motor. The latter produced a correctly wired L6-30R, and it left the cutoff saw alone, which had been working well for the past 30 years or so. That's the route I chose, and things seem to be working well.